1. Field of the Invention
The present invention relates to vapor feed fuel cells with passive thermal-fluids management features. The present invention is particularly useful in applications such as direct methanol fuel cells
2. Description of Related Art
Miniature direct methanol fuel cells (DMFCs), which promise high energy densities and instant refueling, present beneficial opportunities for use as power sources for small mobile devices (e.g., notebook computers, personal digital assistants, music systems and cellular telephones). Although extensive research and industrial efforts have focused on development of miniature DMFCs in recent years, there is still no truly commercial miniature DMFC product available for consumer electronic devices so far. Typically, it has been difficult to provide an ancillary system that ensures effective power generation processes in a miniature DMFC platform. One of the fundamental limitations to faster development of direct methanol fuel cell technology is methanol crossover. Methanol crossover is the process by which methanol is transported, by diffusion and electro-osmosis, from the anode through the electrolyte to the cathode, where it reacts directly with the oxygen, producing no current from the cell. Furthermore, methanol has a poisoning effect on the cathode catalyst that results in reduced cell performance. The methanol crossover rate is roughly proportional to the methanol concentration at the anode; therefore, to reduce methanol crossover, it is necessary to regulate the methanol feed concentration. In practice, the fuel supplied to the anode of the DMFC must be a very dilute aqueous methanol solution (usually 2˜6 vol % methanol). If the methanol concentration is too high, the methanol crossover problem will significantly reduce the efficiency of the fuel cell (Heinzel and Barragan, 1999). This effect exacerbates the difficulties associated with transporting a sufficient water supply to the anode. It is very clear that carrying water in the system significantly reduces the overall system energy density. It is also well known that a forced air design with an external blower is unattractive for use in small fuel cell systems, as the parasitic power losses from the blower are estimated at 20-25% of the total power output.
The conventional approaches to these problems can be divided into two categories: “active” and “passive”. An active system requires moving parts such as a pump or fan to feed fuel and oxygen into the fuel cell stack. Conversely, a passive fuel cell system supplies fuel to the anode in a passive manner requiring no external power or moving parts. A series of active DMFC prototypes has been developed at Motorola Labs (Pavio, 2002; Xie et al., 2004). These systems are composed of the following components: fuel cell stack, methanol sensor, CO2 separator, electronic controls, methanol feed pump, circulation pump and pump drivers. The appropriate methanol concentration is maintained by dosing neat methanol from a methanol cartridge into the recirculation anode liquid. This DMFC design is difficult to create because of the complexity in miniaturizing all the required system components and integrating them into a small unit required for portable applications. In addition, system components add considerable cost to the fuel cell system and consume considerable electricity from the fuel cell, in turn significantly reducing the net power output of the fuel cell. As a result, the actively driven DMFC is not competitive, relative to conventional battery technology, in terms of cost and power output.
One alternative to actively driven systems is passive DMFC systems. Certainly, a number of diverging passive methods have been developed to overcome the difficulties associated with fuel delivery issues, each with its own merits and limitations as shown in Table 1. The passive approaches to this problem can be divided into three categories:
(1) Those utilizing a dilute methanol solution contained in larger fuel reservoirs;
(2) Those utilizing an optimized structure for the fuel cell's electrodes and polymer electrolyte membrane to reduce methanol crossover, permitting use of a highly concentrated methanol solution;
(3) Those utilizing a pervaporation membrane, which effects a phase change from the liquid methanol contained within a fuel reservoir to a vaporous fuel that is presented to the anode aspect of the catalyzed membrane electrolyte.
1. Low Concentrated Methanol Solution Feed (LCMSF)
The simplest method involves utilizing reservoirs containing methanol/water mixtures at the anode (Kim et al., 2004; Han and Park, 2002; Wei et al., 2002). This passive method, has the advantage of system simplicity. It is disadvantageous in that carrying a dilute methanol solution results in a significant penalty to the energy density of the fuel cell. Kim et al. (2004) developed a passive DMFC system having a total active area of 27.0 cm2, which is composed of two planar stacks with a fuel reservoir sandwiched between them. With a 4.0 M aqueous methanol solution, the system produced a power output of 1.0 W at room temperature. Han and Park (2002) used a 4.0 M methanol solution to feed the fuel cell stack. For operation over an extended period of time, they used a large reservoir to eliminate the effect of the changing of methanol concentration. The problem with this approach is that it requires that the system carry a significant amount of water along with the methanol in the cartridge. Carrying a dilute methanol solution in the reservoir, of a composition well under 100% methanol, results in a significant penalty in energy density of the power system.
2. Low Methanol Crossover Polymer Electrolyte Membrane (LMC-PEM)
Toshiba—In U.S. Pat. No. 6,447,941, granted to Toshiba in 2002, the company disclosed a mechanism to deliver liquid methanol fuel by capillary action into the cell. The fuel is then vaporized by heat within the cell and supplied to the fuel electrode, thereby generating electric power (Tomimatsu et al., 2002). The liquid fuel tank is equipped with a pressure control mechanism for introducing a required amount of the liquid fuel from a liquid outlet port into the unit cell.
“The new DMFC adopts a “passive” fuel supply system which feeds methanol directly into the cell. In developing a passive DMFC, Toshiba found a solution to the potential problem of “methanol crossover,” in which methanol and oxygen combine without an energy-producing reaction. The company has optimized the structure of the fuel cell's electrodes and polymer electrolyte membrane that trigger the reaction. This approach allows use of a highly concentrated methanol solution as a fuel, which also overcomes a major obstacle to small fuel cells: achieving a very small fuel tank.”—http://www.pcworld.com/news/article/0,aid,113844,00.asp
Hitachi—“Hitachi's prototype uses a methanol concentration of around 20 percent, although the company plans to raise this to around 30 percent by the time it becomes a commercial product.”—http://www.dpreview.com/news/0406/04062401toshibafuel.asp
Samsung—“A lot of the development work surrounds the membrane at the heart of the fuel cell and the catalyst employed. Miniaturizing the DMFC and extending its life means using a higher concentration of methanol, although that has caused problems with the membrane and some wastage of methanol.
Samsung says its fuel cell uses a new membrane that halts more than 90 percent of methanol crossover and also uses a catalyst made of mesoporous carbon, cutting by half the amount of catalyst required.”—http://www.pcworld.com/news/article/0,aid,115549,00.asp
Chang from Samsung developed a low methanol crossover electrolyte membrane, which maintained the same proton conductivity and near 30% crossover vs. Nafion® when 5M or higher concentrations of methanol were used (Chang, 2003).
3. Phase-Changing Pervaporation Membrane (PCPM)
MTI MicroFuel Cells Inc.—has actively pursued passive fuel delivery technology for micro DMFCs since the company was established in 2001. In the MTI design, a non-porous thin film of silicone is used as a methanol vapor delivery membrane (see FIG. 1A). The silicone thin film used in the MTI invention is a polydimethylsiloxane (PDMS) membrane (Ren et al, 2004a). This membrane has excellent processing ability for making ultra-thin composite membranes. PDMS membranes exhibit selective transport for organic molecules with respect to polar molecules such as water and low molecular weight gases such as nitrogen, oxygen, and helium (Chandak et al., 1998; Hockaday et al., 2003a, b, Thrasher and Rezac, 2004). The permeation of methanol through a PDMS membrane involves three physical processes: (1) sorption of liquid methanol molecules at the feed side of the membrane, (2) diffusion of the dissolved methanol through the membrane, and (3) desorption of methanol vapor from the permeate side. Methanol vapor condenses at the anode and keeps the local methanol concentration next to the catalyzed anode surface at about 3% (1M), or below, which is the concentration level for the anode reaction to proceed with minimal methanol loss due to crossover.
Fuel cell systems based on this type of technology can be completely passive as long as water produced in the system can be reused by means of materials and structures. However, there are two fundamental limitations with this approach. First, as methanol evaporates from the permeate side of the methanol pervaporation membrane, the membrane will cool down. Water will condense on the permeate side of the membrane if its temperature is below the saturation temperature. A thin film of water will be formed on the surface, which will drop the fuel vapor pressure and reduce the rate at which fuel can vaporize out. Secondly, the fuel supply rate is difficult to control. Such controlled adjustment of the rate of liquid fuel delivery is an important key for achieving high fuel utilization in a passive DMFC. To overcome these disadvantages, a new fuel delivery system was proposed by the company (See FIG. 1B). “By using a parallel network of tubes of a sufficiently small diameter, relatively high linear flow of methanol within each narrow tube is achieved at some given overall fuel feed rate demanded by the anode. The linear liquid flow rate could then be made much greater than the linear rate of water diffusing back into the feed tube from any liquid water, which may collect in the evaporation pad during cell operation. This effectively prevents diffusion of water generated at the cell electrode back to the fuel reservoir, which back diffusion, if left unchecked, could result in dilution of the highly concentrated fuel, causing feed of fuel of variable concentration.”—(Ren et al., 2005). This fuel delivery system is actually an actively driven system.
Manhattan Scientifics Inc.—also utilizes “selectively permeable membrane” technology. The membrane is highly permeable to fuel over water. The inventors in U.S. Pat. No. 6,630,266 also called this process as “per-evaporation” (Hockaday et al., 2003a). “The process of enhancing the selective vaporization of fuel from a membrane is called per-evaporation. It essentially increases the evaporation of that fuel. Thus the ampoule membrane uses this effect when the fuel concentration is low. It keeps the fuel concentration higher at the fuel cell than it would be without the fuel ampoule selectively permeable barrier.”
“By having a selectively permeable fuel tank wall, such as silicone rubber, the fuel delivery has the advantageous effect of delivering fuel at a constant rate throughout its life cycle. If the membrane had similar permeability to fuel compared to water, the water would be diffusing in while the fuel was diffusing out. The water would drop the fuel vapor pressure and reduce the rate at which fuel can diffuse out. Thus, the rate of fuel delivery would gradually drop and the fuel tank would gradually fill with a mixture of fuel and water.”
“In product applications it is desirable to have the membrane be effectively much more permeable to fuel compared to water. Thus, it is a “one-way” diffusion process and the rate would not change until the fuel tank is emptied of fuel. Our measurements on silicone rubber membranes show a molecular diffusion rate difference for methanol over water of 20 to 36 times. In performance tests with a small ampoule containing 95% methanol with a silicone rubber membrane the fuel delivery system is effective in delivering fuel with only a small fraction of the original fuel volume left as water in the fuel container.”
While the trend toward passive techniques is becoming an attractive choice for driving small DMFCs, the above-mentioned passive techniques focus mainly on one aspect in the DMFC, namely fuel delivery. Fuel storage, CO2 release, water and thermal management, and orientation-independent operation are some of the other unresolved issues in such systems. In designing a complete power system, these issues must also be addressed. The present invention provides a solution for these problems.